1. Field of the Invention
The present invention relates to a field effect transistor amplifier which is used in a microwave band and which has high gains, especially in a plurality of bands which are separated from each other.
2. Description of the Related Art
FIG. 9 is a circuit diagram of a conventional field effect transistor (hereinunder referred to as "FET") amplifier described in, for example, "GaAs FET Ultrabroad-Band Amplifiers for Gbits/s Data Rate Systems" by K.HONJO et al, IEEE 1981, JULY Trans. on MTT-29. In FIG. 9, the reference numeral 1 represents an FET with the source terminal thereof grounded and having a gate terminal G, a source terminal S and a drain terminal D. The reference numerals 2 and 4 represent transmission lines which are connected in series to resistors 3 and 5, respectively, and provided between the gate terminal G of the FET 1 and the ground and between the drain terminal of the FET 1 and the ground, respectively. The reference numerals 6 and 7 represent matching circuits. One end of the matching circuit 6 is connected to the gate terminal G of the FET 1 and the other end to an input terminal 8, while one end of the matching circuit 7 is connected to the drain terminal D of the FET 1 and the other end to an output terminal 9.
The operation of the conventional FET amplifier will now be explained.
The values of the respective elements are selected so that the influence of the resistors 3, 5 is smaller than that of the transmission lines 2, 4 in a high-frequency band. Therefore, in a high-frequency band, impedance matching is carried out by using the transmission line 2 and the matching circuit 6 on the input side and by using the transmission line 4 and the matching circuit 7 on the output side. On the other hand, in a low-frequency band, it is possible to suppress the increase in the unnecessary gain of the amplifier by making the influence of the resistors 3, 5 larger than that of the transmission lines 2, 4. Thus, according to the FET amplifier, it is possible to obtain a flat gain in a wide band ranging from a low-frequency band to a high-frequency band.
However, it is necessary in the conventional FET amplifier to suppress the gain in a low-frequency band in order to obtain a flat gain in a wide band. For this purpose, it is necessary to provide the resistor 3 between the gate terminal G of the FET 1 and the ground in series with the transmission line 2. It is therefore impossible to completely disregard the influence of the resistor 3 in a high-frequency band, so that the gain is disadvantageously reduced. In addition, in order to obtain gains in two bands which are separated from each other by several octaves, since the gain is greatly reduced by the resistor 3 in the conventional FET amplifier so as to make the gain characteristic flat in a wide frequency band which covers the two bands, it is very difficult to obtain high gains only in the two desired bands.